1. Field of the Invention
The present invention relates to a steam generator, more particularly to a steam generator capable of generating steam in a short period of time. Further, the present invention relates to a mixer using the above-mentioned steam generator, more particularly to a mixer which generates a mixed gas by mixing steam with another material.
2. Description of the Related Art
A steam generating apparatus is used for a steam type inhaler which sucks up and sprays a liquid by means of using discharge of steam, an exhaust gas purifying apparatus as will be described later, etc., for example.
Firstly, the steam generating apparatus used for the steam type inhaler will be described. In this type of steam type inhaler, a nozzle which discharges the steam is provided and an upper end of a sucking tube whose other end is put in a liquid within a liquid tank is disposed in the vicinity of the above-mentioned nozzle. Water is heated to generate steam. When the steam is discharged through the nozzle, a liquid will be sucked up through the sucking tube and atomized along a steam flow as the steam is discharged.
In this case, in order to shorten a time period between start of heating and start of atomizing after the steam is generated, the water in a feed water tank is not directly heated, but a heat chamber of small capacity is provided separately, and part of water in the feed water tank is led to the heat chamber in which steam is generated.
For more detailed description, an example will be described. For example, a steam suction apparatus proposed in Japanese Laid Open Patent No. 2000-126296 will be described with reference to FIG. 34.
Reference numeral 101 in the figure depicts a feed water tank portion. Reference numeral 103 denotes a heat chamber provided with a heater 102 on its wall surface. A bottom part of a feed liquid tank portion 101 and a bottom part of a heat chamber 103 are communicated with each other through a conductive tube passage 104. An upper space of the heat chamber 103 and an upper space of the feed liquid tank portion 101 are communicated with each other through a steam passage hole (not shown). The steam which is generated in the heat chamber 103 and flowed into the upper space of the feed liquid tank portion 101 is adapted to be directed to a nozzle 105.
In addition, reference numeral 106 in the figure denotes a sucking tube, one end of which is disposed in the liquid tank; reference numeral 107 depicts a relief valve which opens when a steam pressure in the feed liquid tank portion 101 increases, so as to release the pressure; reference numeral 108 indicates a lid which closes an upper end opening of the feed liquid tank portion 101.
In this steam suction apparatus, water is supplied from the feed liquid tank portion 101 to the heating unit 103; the steam generated in this heating unit 103 flows into the upper space of the feed liquid tank portion 101. Then, the above-mentioned steam is supplied from the upper space of the feed liquid tank portion 101 to the nozzle 105, and is discharged through the nozzle 105. In this way the upper end of the sucking tube 106 is caused to have a negative pressure, so that the liquid is sucked up through the sucking tube 106 and is atomized along the steam flow.
A disadvantage of such an apparatus is that the steam generating apparatus used for the steam suction apparatus is generally made of metal, such as iron and aluminum and heat from the heater 102 escapes to a body (casing) of the apparatus, so that the water in the heating unit 103 cannot be heated effectively, the thermal efficiency is low, and it is not possible to perform rapid heating. The document does not disclose how to solve a problem that the time between the start of heating and the start of atomizing after the steam is generated.
Moreover, the heater 102 is arranged on a wall surface of the heat chamber 103 in the above-mentioned steam generating apparatus so that the water may be heated directly. Therefore, in order to prevent leakage, it is necessary to provide a member, such as an 0 ring 109, between the heater 102 and the wall surface, and to maintain the member according to used hours.
Furthermore, in the above-mentioned steam generating apparatus, the feed liquid tank portion 101 and the heat chamber 103 are connected by pipes, the upper space of the heat chamber 103 and the upper space of the feed water tank are communicated with each other through the steam passage hole, and the feed liquid tank portion 101 and the nozzle are connected by a pipe. Thus, there is a problem that the whole apparatus is complicated and its manufacturing costs increase.
The steam generating apparatus is used also in an exhaust gas purifying apparatus. Now, we describe an example of this exhaust gas purifying apparatus. For example, in Japanese Laid Open Patent No. H05-272331, as shown in FIG. 35, an exhaust gas purifying apparatus is disclosed which comprises an NOx reduction catalyst 151 disposed in the middle of an exhaust pipe of a diesel engine 150, a reducing agent feed nozzle 154 disposed on an upstream side of the NOx reduction catalyst 151 of an exhaust pipe 153, a reduction gas generator 155 connected to the reducing agent feed nozzle 154, a urea supply device 157 which supplies solid urea 156 to the reduction gas generator 155, and an air supply device 158 which supplies air to the reduction gas generator 155.
This exhaust gas purifying apparatus thermally decomposes the solid urea 156 whilst being supplied with air, so as to generate a reduction gas which is supplied to an exhaust gas channel upstream of the NOx reduction catalyst 151.
In other words, the above-mentioned reduction gas generator 155 is equivalent to the above-mentioned steam generator, and is a mixer in terms of mixing the reduction gas with air.
Now, describing it more particularly, the above-mentioned reduction gas generator 155 comprises an outer casing 159, an air inlet 160 and a reduction gas outlet 161, a flue 162 concentrically arranged within the outer casing 159, and an electric heater 163 arranged around this flue 162.
In addition, the urea supply device 157 comprises a tank 164 and a screw feeder 165. This tank 164 is manufactured to be of a “hopper” type, at an upper end of which an inlet 166 is provided, and at a lower end of which an outlet 167 is provided. The tank 164 is filled with the solid urea 156 which is ground in advance. Moreover, the above-mentioned screw feeder 165 comprises a screw case 168, a screw 169 disposed in the above-mentioned screw case 168, and a motor 170. Furthermore, the above-mentioned air supply apparatus 158 comprises an air tank 172 connected to the air inlet 160 of the flue 162 by means of piping 171, and an air compressor (not shown) connected to the air tank 172.
In the thus constituted reduction gas (thermal decomposition gas) generating apparatus, a screw 169 is rotated and the powdery solid urea 156 is supplied from a tank 164 into the flue 162. On the other hand, while the inside of the above-mentioned flue 162 is heated by the electric heater 163, air is supplied from the air tank 172.
As a result, the solid urea 156 is thermally decomposed so as to be a reduction gas, which is conveyed to the reducing agent feed nozzle 154 by means of the supplied air.
A reduction gas generator (mixer) used for the above-mentioned purifying apparatus is constructed such that the solid urea is supplied to the inside of the flue so a to be heated inside the flue. Therefore, there is a technical problem that the solid urea should be rapidly heated inside the flue, so that a large-sized heater should be used.
Moreover, the air supply device is used for generating and conveying the above-mentioned thermal decomposition gas (reduction gas), however, there is another technical problem that a temperature in the flue is decreased by the air supply, so that the thermal efficiency is reduced.
Furthermore, the reduction gas generator (mixer) used for the above-mentioned purifying apparatus, the urea supply device, and the air supply device are each formed separately; the urea is adapted to be supplied by the screw feeder; and the air is adapted to be supplied from an air tank. Therefore, there is a technical problem that the whole apparatus becomes larger.
Furthermore, in the above-mentioned purifying apparatus, compressed air is used as a means to pump the reduction gas to an exhaust-gas passage, so that an apparatus, such as an air tank for storing the compressed, a compressor, etc. is needed. Thus, there is another technical problem that the apparatus becomes larger and more complicated so that it is difficult to load it on a vehicle.
In addition, the flue etc. might corrode by ammonia generated when the solid urea is thermally decomposed. In order to solve this, it may be possible lo apply a Teflon (registered trademark) coating inside the flue. However, there is another technical problem that the application of the coating may be labor intensive and expensive.